1. Field of the Invention
The present invention releates to a method for preparing electrically-conductive needle-like zinc oxide and more specifically to a method for preparing electrically-conductive needle-like zinc oxide which permits, at low cost and high efficiency, the preparation of the electrically-conductive needle-like zinc oxide which is highly pure, which has a high whiteness, a three-dimensionally branched needle-like shape and a low powder specific resistance of little dispersion because a dopant is uniformly dispersed.
2. Description of the Prior Art
Zinc oxides are useful as, for instance, pigments kneadable with paints and varnishes, resins, rubbers and fibers and there has been desired for the development of zinc oxides having excellent electrically-conductivity, in particular, in antistatic applications. Among these zinc oxides, those having unbranched needle-like or branched needle-like (tetrapod-like) shapes (hereinafter generically referred to as "needle-like zinc oxides") are excellent in that a electrically-conductive network can be constructed through the addition thereof in an amount smaller than that required for granular zinc oxide. For this reason, there has been required for the development of zinc oxide having a high electrically-conductivity and a needle-like shape.
As a method for preparing needle-like zinc oxide, in particular, three-dimensionally branched (tetrapod-like) zinc oxide, there has been known the so-called "American Method" which comprises oxidizing zinc vapor containing a reducing gas. However, the resulting zinc oxide has a high powder specific resistance and is a mixture with conglomerates thereof or the resulting zinc oxide contains, for instance, lead and cadmium in an amount on the order of 1000 ppm, even if it substantially comprises needle-like zinc oxide. Thus, the product obtained according to this method is not favorable for use as an auxiliary and an additive.
Moreover, there have also been known other methods such as a method comprising subjecting metal zinc vapor combustion and oxidation and quenching the resulting zinc oxide immediately after the combustion and oxidation (see Japanese Unexamined Patent Publication (hereinafter referred to as "J.P. KOKAI") No. Sho 56-120518); a method comprising heat-treating oxidized layer-containing powdery zinc in an oxygen-containing atmosphere to give zinc oxide (see J.P. KOKAI No. Hei 1-252600); a method comprising neutralizing a solution containing an alkali metal zincate compound and a salt of a specific metal such as aluminum to thus co-precipitate these salts and then calcining the co-precipitates in a reducing atmosphere (see J.P. KOKAI No. Hei 3-28125); a method comprising adding an ammonium carbonate aqueous solution and an aluminum sulfate aqueous solution to a dispersion of needle-like zinc oxide, then dehydrating the mixture after stirring over a predetermined time period and calcining the dehydrated product in a reducing atmosphere (see J.P. KOKAI No. Hei 3-60429); and a method comprising vaporizing zinc powder or zinc metal by indirectly heating it to thus oxidize the vaporized zinc metal (zinc vapor) (see J.P. KOKAI No. Hei 3-150299). However, these methods are not suitable for mass-production of desired zinc oxide. Further they require the use of large amounts of alkalis and large amounts of acids and this becomes a cause of a problem concerning production facilities and results in low production efficiency. They include calcination processes carried out in reducing atmospheres and, therefore, the resulting electrically-conductive needle-like zinc oxide has a tinge of grayish color or an insufficient whiteness. In addition, the powder specific resistances thereof are high and are not satisfactorily stable since the resulting products do not comprise any dopant. Moreover, the electrically-conductivity of the resulting needle-like zinc oxide is still insufficient and there remains a room for further improvement.